A great variety of bleaching and cleaning compositions have been described in the art based on persulfate salts as a bleaching agent. Furthermore, it is highly desirable to add other ingredients to such bleaching compositions such as surfactants in order to improve the detergency effect of said compositions. However, persulfate salts based compositions in aqueous liquid form have the drawback that they are not stable. Indeed, such liquid bleaching compositions based on persulfate salts decompose in the presence of other ingredients in said compositions, such as surfactants, that can be oxidized by the persulfate salts. Also persulfate salts decomposition can be catalyzed by contaminants, e.g., heavy metal ions. As a result said compositions tend to be chemically unstable and consequently tend to lose their cleaning and bleaching capacities.
It is therefore an object of the present invention to provide aqueous compositions comprising persulfate salts which are chemically stable, especially upon prolonged periods of time.
It has now been found that the stability of an aqueous persulfate salts based composition, can be improved by selecting a very narrow pH range as well as by adding a radical scavenger thereto. Indeed, the presence of both conditions, a very narrow pH range, i.e. of from 0 to 0.9, and the addition of a radical scavenger, or mixtures thereof, are necessary to achieve the desired chemical stability which will enable an aqueous persulfate salts based composition to be marketed.
As a further advantage it has been found that the compositions of the present invention having a pH of from 0 to 0.9, comprising a surfactant, e.g. a nonionic surfactant, a radical scavenger and a persulfate salt, find a preferred application in the cleaning of toilet bowls. Indeed, once a composition according to the present invention has been applied onto the top of the inclined surface of a toilet bowl, said composition slowly reaches the water, where it is diluted and thereby becomes less acidic, i.e. from about pH 0.5 to about pH 2. The compositions according to the present invention have enhanced cleaning performance in their dilute form, compared to their neat form. Actually, this "pH jump" effect allows to formulate compositions which are stable upon storage (with a pH of from 0 to 0.9) and effective in cleaning. Additionally, the compositions of the present invention exhibit good soil solubilization and soil discolouration properties both when used neat and diluted.
EP-B-209 228 discloses liquid compositions having a pH of from 1 to 8 and comprising nonionic surfactants, a peroxide compound and substituted mono- and di-hydroxybenzenes such as BHT as the radical scavenger. The '228 patent mentions that the peroxide compound is normally present as hydrogen peroxide. Persulfate salts are not disclosed.
EP-A-340 001 discloses a non-aqueous liquid composition comprising radical scavengers such as BHT and a bleach (halogen or oxygen bleach). Perborate is preferred and persulfate salts are not disclosed per se. No pH is disclosed.
EP-A-199 385 discloses compositions having a pH below 4, which comprises a peroxy compound, anionic and nonionic surfactants. No radical scavengers are disclosed.